Dynamoelectric machine control circuit having current limiting means

ABSTRACT

A D.C. shunt motor armature is connected to a three-phase A.C. input through a pair of parallel-connected, full-wave, silicon controlled bridge rectifiers to selectively vary the voltage and current supply to the armature. The controlled rectifiers are phase controlled in accordance with a command voltage, an armature voltage and a modifying voltage proportional to the counterelectromotive force for IR compensation and armature current for current limit. The modifying voltage includes an armature current related signal establishing a percentage armature current compensation which is of the order of 50 percent and greatly in excess of the normal IR compensation and directly establishes a continuous current limit to positively prevent excessive armature current by phasing back of the bridge rectifiers.

United States Patent Maynard [54] DYNAMOELECTRIC MACHINE CONTROL CIRCUITHAVING CURRENT LIMITING MEANS [72] Inventor: John T. Maynard, NewBerlin, Wis.

[73] Assignee: A. O. Smith Corporation, Milwaukee,Wis.

[22] Filed: May 20, 1970 [21] App]. No.: 39,016

[4 1 Aug. 8, 1972 2,929,979 3/1960 Fischer .(318/345 PrimaryExaminer-Bernard A. Gilheany Assistant ExaminerThomas LangerAttorney-Andrus, Sceales, Starke & Sawall [57] ABSTRACT A DC. shuntmotor armature is connected to a threephase AC. input through a pairofparallel-connected, full-wave, silicon controlled bridge rectifiers toselectively vary the voltage and current supply to the armature. Thecontrolled rectifiers are phase controlled in accordance with a commandvoltage, an armature voltage and a modifying voltage proportional 'tothe counterelectromotive force for IR compensation and armature currentfor current limit. The modifying voltage includes an armature currentrelated signal establishing a percentage armature current compensationwhich is of the order of 50 percent and greatly in excess of the normal[R compensation and directly establishes a continuous current limit topositively 13 Claims, 4 Drawing Figures 5 .Va ISOLATlNG &

52 US. Cl .J. ..318/331, 318/345 [51] Int. Cl. ..H02p 5/16 58 Field ofSearch ..318/327, 331, 332, 345

[56] References Cited UNITED STATES PATENTS 3,284,688 11/1966 Black..318/332 3,497,786 2/1970 Lombardo ..318/331 3,181,050 4/1965 Berman..318/331 2,847,632 8/1958 Harvey ...3l8/327 3,252,069 5/1966 Ringrose....3l8/331 3,494,340 1/1950 Leigh ....3l8/345 2,845,589 7/1958 Osgood..318/345 COMMAND SIGNAL REFERENCE "TRANSFORMER GATING FILTERING CIRCUITPMENIEM: awn 3.683252 SHEET 2 0F 2 INVENTOR. I JOHN T MAYNARD Ajay/Attorneys DYNAMOELECTRIC MACHINE CONTROL CIRCUIT HAVING CURRENT LIMITINGMEANS BACKGROUND OF THE INVENTION Direct current motor drives and thelike advantageously employ a feedback system to control the input powerto the motor and thereby the motor speed output or the like. A directcurrent shunt motor provides a highly desirable drive where a relativelyconstant speed combined with a high starting torque is 1 required. Sucha motor may have a fixed field excitation with a variable armaturecurrent control for establishing the desired speed and torquecharacteristic. A highly satisfactory system for controlling a directcurrent shunt motor or the like is shown in applicants copendingapplication Ser. No. 713,247, filed Mar. 14, 1968 and titledDynamoelectric Control Circuit, wherein an analog control systemcompares a DC. command signal with a plurality of correspondinglyrelated feedback signals. The analog feedback system or the controlsystem employs phase control of a plurality of controlled rectifiersconnecting the armature to a three-phase power supply. Firing iscontrolled by the intercept between an alternating control signalrelated to the voltage applied to the control rectifier and a directcurrent analog intercept firing signal. In particular, the system sums acommand voltage with a feedback armature voltage to establish an errorsignal. This amplified error signal is further modified by a voltageproportional to the counterelectromotive force (CEMF or counter EMF) ofthe motor armature to provide for automatic tracking of the zerocrossing point. The summation or modification of the error signal by theCEMF signal provides an effective zero gating for zero armature currentwhile continuously adjusting the reference level from which the errorsignal operates to control the firing in accordance with the actualerror signals and the CEMF voltage.

In such systems, it is important, however, that the armature current belimited. A convenient circuit to provide current limit is avoltage-sensitive diode, such as a Zener diode connected in the currentfeedback network. At an excessive current, the diode abruptly breaksover and conducts, thereby increasing the current feedback to, inessence, swamp the control signal and phase back the firing. Althoughthis system is highly desirable for any given motor, applicant found thearmature current versus error signal characteristic varied from motor tomotor and in some instances a condition of possible instability wascreated. The prior circuit also employed an input or supply lineisolation transformer which tended to stabilize the current responsecharacteristic. However, variations of transformer and source impedancealso contributed to the variation in the particular error signalcharacteristic. However, with the transformer impedance removed, Zenerdiode type protection was absolutely essential to prevent excessivecurrent. Generally, the current versus error signal characteristic witha Zener diode has three distinct portions, including an initial smallslope portion corresponding to a discontinuous armature current region,followed by a very steep slope portion corresponding to a continuousarmature current region covering the regulating voltage range and afinal Zener voltage breakover or current limit range. In this latterrange, the effective armature current limit value is dependent upon themagnitude of error voltage when Zener breakover occurs and the currentlimit deviation from set point (15 percent rated current typical) may beas great as percent over the error voltage range. Thus, the shape of thecurve was essentially an S- shaped curve with very sharp areas oftransition. The steep regulating portion, however, constituted a verysmall percentage, typically 15 percent of the total error voltageresponse and thus, any variation in the characteristic resulted in acondition which would tend to create instability, particularly in thetransition regions.

SUMMARY OF THE PRESENT INVENTlON The present invention particularlyrelates to a dynamoelectric control circuit for controlling the armaturecurrent or the like and particularly to such a circuit employing acontinuous and improved current limit control.

Generally, in accordance with the present invention, an error signalindicative of the difference between the desired motor operation and theactual motor operation and an armature current feedback signal isestablished as a part of a summated control signal with the relativepercentage of the error signal and the current-related feedback signalinterrelated to establish what would normally be considered excessivecurrent related feedback compensation. Applicants analysis of thecircuit problem indicated that by establishing a relatively excessivecompensating feedback, the armature current versus error signalcharacteristic of the motor control could be corrected to establish andmaintain stable and improved operation with very accurate and reliablecurrent limit control. Thus, the transfer function or characteristic ofthe control system is such that the response is essentially the same,notwithstanding the slight variations inherent in the required componenttolerances for commercial production of various motors and the relatedcontrols. Further applicant has found that the current feedback signalis applied directly as an essentially unfiltered current signal and inphase with the main current. This provides the desired DC. signal with asuperimposed ripple which is, in essence, in phase with the mainarmature current. Applicant has found that the unfiltered, in-phasecurrent automatically retards and advances the firing point to maintainvery reliable modulation of the firing point.

In accordance with a further teaching of the present invention, currentlimit control can be established by changing of the relative level ofthe current feedback current and the error signal. As the two signalsoperate in opposition, the armature compensating current signal can beincreased or the error signal can be reduced or starved. The firing orgating signal'is thus made up of a plurality of current-relatedvoltages: a current signal which retards the firing point and an errorsignal which advances the firing point.

The present invention appears to establish or derive the substantial andincreased stability from a shaping of the transfer function and inparticular the elimination of all sharp transitions in thecharacteristic while providing a well defined cutoff within thedesirable current limit ranges. The present invention thus provides ahighly improved current modulation and stability in the motor responsecharacteristic while maintaining reliable armature current limitinherently independent of motor and source impedance parameters.

BRIEF DESCRIPTION OF THE DRAWINGS The drawings furnished herewithillustrate a preferred construction of the present invention in whichthe above advantages and features are clearly disclosed, as well asothers which will be clear from the following description.

In the drawings:

FIG. 1 is a diagrammatic view of a motor control circuit constructed inaccordance with the present invention;

FIG. 2 is a diagrammatic graphical illustration of the control voltagesin the circuit of FIG. 1;

FIG. 3 is a diagrammatic graphical illustration showing the armaturecurrent versus error signal characteristics of a Zener diode currentlimit control circuit; and

FIG. 4 is a similar diagrammatic illustration of the characteristiccurve resulting from the present invention.

Referring to the drawings, and particularly to FIG. 1, the presentinvention is illustrated as applied to control the operation of a directcurrent shunt motor 1 of any well-known construction and generally inaccordance with the teaching of applicants previously identifiedcopending application. The illustrated shunt motor 1 is diagrammaticallyshown including a field 2 connected to a fixed D.C. excitation source 3.The motor 1 includes the usual armature 4 which is rotatably mountedwithin the field 2. Armature 4 is connected to a direct current powersupply circuit 5 including a pair of gated rectifier bridge networks 6and 7 for selectively providing a forward and reverse current to thearmature. The input sides of the bridge networks 6 are connected inparallel to a suitable three-phase power source and may, in accordancewith the teaching of the present invention, be connected directly to thewidely employed industrial 460-volt alternating current supply lines 8,without the necessity of a line coupling transformer. The bridgenetworks 6 and 7 are well-known, threephase, full-wave bridge networkswhich, in accordance with the present invention, use an individualsiliconcontrolled rectifier 9 in each leg of the bridge. The controlledrectifiers 9 are interconnected to a digital gating regulator 10 toselectively control the firing and conductivity of the rectifiers. Therectifier networks 6 and 7 are reversely connected such that the onerectifier network 6 establishes a given directional flow with acorresponding torque, and the opposite bridge network 6 provides anopposite directional torque. Either bridge network will provide motoringor regeneration, depending upon the load and the firing of rectifiers 9.The digital gating regulator 10 functions in response to the interceptof a DC. signal with an alternating current reference signal, inaccordance with the teaching of applicants copending application. Thealternating current reference signal is derived from a referencetransformer 11 connected to the three-phase power transformer 8. Thereference signal is modified with respect to actual rectifier anodevoltage trace and is generally of an opposite phase for the particularlogic transistors employed. Thus, the circuit preferably employs anNPN-transistor circuit with a positive logic. Referring particularly toFIG. 2, the controlled rectifier anode voltage is shown by trace 12. Thefiring reference voltage 13 is inverted, and is phase-shifted 30 degreesleading to establish a maximum trigger in synchronism with the angle ofthe anode voltage trace 12. An initial D.C. level bias signal, shown bythe level line 14, is superimposed on the reference signal and ineffect, shifts the reference signal with reference to the anode voltagesuch that the reference signal intercepts the DC bias level signal insynchronism with the zero crossover point of the actual anode voltagetrace 12.

As noted above, the reference signal is phase-shifted 30 such that themaximum level signal of the reference signal coincides with the 60 angleof the actual anode voltage. This establishes the maximum advance firingpoint which, in a three-phase system, corresponds to the firing pointfor maximum possible output. Further, it permits control extending fromthe 60 firing for motoring through 60 during the negative half-cycle forpurposes of regenerating.

The DC bias signal level 14 is modified in accordance with feedbackvoltage signals, as presently described, to advance or retard the firingpoints in accordance with motoring and regenerating operation.

Referring again to FIG. 1, the illustrated control circuit includes ahigh-gain error amplifier 15 having a first D.C. input connected to acommand signal source 16 and a second input signal terminal which isconnected to an armature voltage feedback line 17. The present inventionemploys a suitable scaled control voltage. Thus, the control commandsignal may vary, for example, between zero and 10 volts which will berelated to a variation in the armature voltage of zero and 500 volts.The voltage feedback line 17 is interconnected to a voltage isolatingand filtering circuit 18 connected directly to the motor armature 4.Circuit 18 provides an isolated, essentially constant direct current(d.c.) signal proportional to the actual armature voltage, but scaled tothe command voltage. The error amplifier 15 is selected to have asubstantial gain and the output is proportional to the amplifieddifference in the command voltage signal and the feedback voltage signalmultiplied by such error amplifier gain. A resistor-capacitor feedbacknetwork 19 may be employed to stabilize the amplifier 15 in accordancewith known design. The output of the error signal is applied to asimilar summing amplifier 20 where it is modified by acounterelectromotive force-related signal and withan automatic currentlimit such that the DC. bias level firing signal is a summation of theerror signal and the several compensation signals. Generally, firingvoltage level V, can be written in equation form as:

wherein the BAV is the error signal, V is the scaled armature voltage,V, is the scaled armature current, hereinafter referred to as thecurrent feedback I,,, R, is the scaled armature resistance, and V, isthe DC. bias level. The delta V and the armature voltage V have anegative sign indicating that they tend to advance the firing angle andthereby increase the armature voltage and correspondingly the armaturecurrent. The plus sign associated with the other compensating factorsand, in particular, the armature current and the DC. bias indicate thatthey tend to retard the firing angle and thereby reduce the currentsupply to the armature. As hereinafter developed, the present inventionemploys adjustment of the error signal, delta V and/or the currentfeedback I in combination with a substantial current feedback to providecontinuous current limit with improved stability of response.

More particularly referring to the circuit of FIG. 1, the armaturevoltage signal is derived through the interconnection of an armaturevoltage line 21 to the voltage isolating and filtering network 18. Line21 is connected to the input of the summing amplifier through a pair ofseries-connected resistors 22 and 23, the centerjunction ofwhich isconnected via a noise filtering capacitor 24 to logic common line 25. Aresistor 26 connects the common line to the opposite side of theamplifier 20. This provides an essentially constant D.C. signalproportional to the actual armature voltage to the summing amplifier 20.

The D.C. bias voltage is supplied to the input of the summing amplifierfrom a voltage source line 27 through a pair of series-connectedresistors 28, the common junction of which is interconnected to thelogic common line 25 by a capacitor 29. The D.C. bias voltage levelprovides a constant input signal to the amplifier 20 establishing thedesired D.C. output signal in the absence of all other input signals.

In accordance with the present invention, the armature current signal isinterconnected into the summing amplifier 20. The current sensingnetwork 30 may, for example, include current transformers 31 and athreephase, full-wave rectifier 32, establishing a properly scaledcurrent signal. Of particular significance is the direct connection ofthe output of rectifier 32 without any significant filtering toamplifier 20. A pair of seriesconnected resistors 33 interconnect thecurrent line 28 directly to the input of the amplifier 20. A very smallcapacitor 34 interconnects the junction of the resistors 33 to the logiccommon line 25. Capacitor 34 will provide transient protection, but willnot produce any significant filtering. This inserts an armature currentsignal which is essentially a D.C. voltage signal, including the ripplecomponent of the armature current as applied to the armature.

In addition to the above, the output of the error amplifier 15 isconnected to the summing amplifier 20. The interconnecting erroramplifying network includes a pair of series-connected resistors 35 and36 connected between the output of the error amplifier l5 and the inputof the summing amplifier 20. A transient filtering capacitor 37interconnects the junction of the resistors 35 and 36 to logic common25. In accordance with the present invention, the error amplifierconnecting network includes a current limit setting potentiometer 38interconnected in series between amplifier 15 and the couplingresistors. The current limit potentiometer 38 includes the movable tap39 for selectively bypassing portions of the current limit resistor 40of potentiometer 38. This, therefore, adjusts the current supplied tothe summing amplifier 20 with changes in the error signal delta V and aspresently described, shifts the transfer function and characteristic ofthe closed-loop system. The illustrated summing amplifier 20 has a pairof parallel-connected, oppositely polarized clamping diodes 40aconnected across the input terminals and an R-C stabilizing feedbacknetwork 40b connected between the input and the output to produce thedesired response to rapidly changing signals.

Referring to FIG. 2, the delta V signal is thus summated with the D.C.bias level line 14 to establish a new intercept line 41. Furthersuperimposed on this line 41 is the effect of the voltage feedback viaresistors 22 and 23 and the armature current feedback via the resistors33. The voltage signal is a suitable filtered, direct current levelwhich functions in the same direction as the error signal to establish aselected constant D.C. level line 42. This level is further changed bythe current feedback signal, which in accordance with the presentinvention, is an unfiltered direct current signal shown as trace 43added to and modulating the intercept line 42. The current feedbacksignal trace 43 includes both the l and I R component of the previousequation. The I R component relates to the voltage by which the armaturevoltage V, is reduced to establish a signal proportional to thecounterelectromotive armature voltage. This is generally of the order of10 to 11 percent maximum of the rated voltage as scaled to the voltagelevel of the control system. However, the present invention employs acurrent feedback signal which can be similarly related as feedbackvoltage in terms of the rated voltage in excess of this and preferablyof at least the order of 30 or percent, of the rated voltage which isindicated in the equation by the voltage signal related to factor IApplicant has discovered that the substantial and excess feedbackstabilizes the operation of the circuit and permits operation withoutthe usual line source coupling transformer employed heretofore in orderto introduce a stabilizing impedance factor.

Referring particularly to FIG. 3, a characteristic curve indicating thetransfer function of the system and particularly relating the change inarmature current signal I with a change in error signal AV isillustrated for the control circuits heretofore constructed employing aZener diode current limit. In this device, the armature current wasinterconnected through a suitable coupling transformer and well filteredto establish a constant D.C. proportional to the average armaturecurrent. The current control range existed approximately over zero to IOvolts, with an initial relatively low-slope portion 44 covering fromzero to 6 volts. At that point, a relatively sharp break in the curvewas established for a relatively slight change in voltage as at 45 witha second high slope portion 46 which was essentially linear, thusdefining a discontinuous characteristic. Generally, the voltage controlof the second portion 46 covered about 2 volts before the maximumpermissible current level was reached.

Thus the characteristic of FIG. 3 shows the overall motor armature anderror signal voltage characteristic. The portion 44 of the curve isgenerally related to a discontinuous current characteristic wherein thecurrent supplied to the armature for each input cycle of the powersupply is a series of time spaced pulses with a significant zero currentbetween pulses. The illustrated current of FIG. 3 is the average currentsupplied by such time spaced pulses. This area is generally known as adiscontinuous current area. As the current supply is increased thepulses increase in amplitude and width until such point as thetermination of one pulse at the zero axis is associated with essentiallythe immediate initiation of a subsequent pulse. At this particulartransition point the characteristic changes, generally as shown by thecurved portion 45, which defines a transition area to the continuouscurrent characteristic of portion 46. In the latter, the current pulsessignificantly overlap and another pulse starts before any given pulseengages the zero axis. In this area the current changes relativelyrapidly with the error voltage signal (V) and thus according to theslope of line 46 which once again is the average of the overlappingpulses. Thus as the error voltage signal changes, the current rapidlyincreases. In accordance with the characteristic of FIG. 3, the currentrises to a point 46a which is approximately the desired maximum armaturecurrent for the particular circuit. At approximately permissible currentlevel, the circuit was constructed such that the Zener diode broke overto conduct as the point 46a and establish a direct and essentially Ipercent current feedback. This high current feedback, represented by theseparate I, in the equation and particularly the voltage related signalin the voltage equation, significantly reduced the slope and resulted ina current limit which was dependent upon the error voltage magnitude. Aspreviously noted, although this circuit provides a satisfactorilyoperating system, the particular characteristic curve would changeslightly with the error voltage, the particular motor and the particularsource transformer, for example, as shown by the dashed lined curves 47and 48. Further, when the source transformer was removed, the transferfunction slope 46 in creased to further restrict the control range.Stable operation is difficult to achieve under these conditions.

In accordance with the present invention, the armature current signal isfed back without any essential filtering and without the interpositionof an iron core source transformer. Nevertheless, the circuit operatessatisfactorily with an automatic current limit. It appears that thetransfer function or characteristic of FIG. 3 is modified to that showngenerally in FIG. 4, where the changes of current I with the errorsignal AV is a continuously changing function and, in particular,without any sharp transition points or areas such as defined by thediscontinuous characteristic or transfer function of FIG. 3 and withinterception of the maximum feedback error signal within the desiredcurrent limit. Two curves 49 and 50 of a continuous transfer functionare shown for typical minimum and maximum current limit settings. Thus,by adjusting of the tap 39 of the potentiometer 38, the control circuitcan be made to respond and establish a maximum annature current ofanywhere between 75 and 150 percent of rated armature current. The sameadjustment could, of course, be inserted in line 28, with the settingbeing in an opposite direction in view of the opposite signs of thefactor in the equation.

As noted previously, the current feedback is substantially greater thanthat required for armature voltage drop IR compensation. Thus, normally,armature voltage compensation can require and vary up to 8 percent ofrated voltage and in order to ensure stability, 10 to l I percent ofrated voltage was normally employed. In accordance with the presentinvention, however, applicant has found that the feedback should beincreased to the order of 50 percent and that it will operatesatisfactorily with any feedback up to the order of percent. This degreeof feedback increase modifies the overall characteristic tosignificantly change the slope and in fact tends to straighten andlengthen the transfer function as shown in FIG. 3 to that shown in FIG.4. It would appear that the feedback controls the linearity and the gaincharacteristic of the control system to thereby modify its transferfunction to the relatively smooth and continuously changingcharacteristic with the highly desirable continuous control without the.necessity for other stability devices and the like.

Generally, increasing of the feedback to 15 percent and above, tends toextend or flatten the curvature of the response characteristic as shownin FIG. 4 with a reduction in the maximum percentage current ratingwhich can be employed.

Furthermore, the use of the unfiltered current feedback provides aninherent and automatic regulation or modulation of the firing. Thus, asshown in FIG. 2, the current feedback signal is not a pure DC. signal,but is a full-wave unfiltered signal 43 defining a plurality of hillsand valleys, with the bottom points 51 of the valleys generallyestablishing and corresponding to the firing points. If, for any reason,a given phase is excessive, for example, as a result of an unbalancedsupply line, the corresponding phase current signal will becorrespondingly enlarged as at 52. This will automatically shift thevalley point and tend to retard the firing of the next stage of therectifier 9, thereby reducing the current supplied to the armature andproviding the desired modulation.

Thus, motor specification generally limits the maximum armature currentmodulation or overshoot to a given percentage for a normal supply linevariation. Generally, the specification may be a 10 percent variationfor the normal maximum line voltage change. The present inventionpermits a substantially smaller percent modulation, such as in the orderof 3 to 5 percent and, by accurate current feedback, essentially anabsolute limit on the maximum armature current, such as a 5 percentmaximum deviation.

The adjustment of the particular characteristic can, within limits, bemade either by adjusting the current limit feedback circuit or the errorsignal feedback circuit through the use of a variable impedance element.In the illustrated embodiment of the invention, applicant has shown thepotentiometer 38 in the error signal feedback line which has been usedsatisfactorily to vary and adjust the maximum and minimum limits ofcurrent within the rated current range.

The present invention thus provides an improved motor control systememploying an armature current transfer characteristic without any sharpdemarcation as a result of employing a greater amount of currentfeedback than that ordinarily dictated by armature impedance drop. Thisestablishes a highly improved motor control circuit.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims which particularly point outand distinctly claim the subject matter which is regarded as theinvention.

I claim:

1. A motor control circuit for operating a motor having an armature at aselected rated voltage comprising, first means to establish an errorsignal proportional to the difference between a desired motor operationat a selected rated voltage and the actual motor operation, second meansto establish a compensating signal proportional to thecounterelectromotive force of the armature of the motor, said last namedmeans including negative current feedback means establishing acompensating voltage signal directly proportional to the armaturecurrent and forming a single combination circuit impedance compensatingsignal and an armature current limit signal, said compensating voltagesignal being correspondingly scaled to the error signal and being equalto at least 30 percent of the corresponding scaled rated voltage toestablish and maintain a continuous and smoothly changing transferfunction including said compensating voltage signal, and motor controlmeans responsive to said error signal and to said compensating signal.

2. The motor control circuit of claim 1, having supply lines connectedto said motor and said current feedback means establishes said transferfunction which is essentially independent of the resistance of the motorarmature, the impedance of the supply lines and motor speed.

3. The motor control circuit of claim 1, including three-phase powerdistribution lines, and rectifying means connecting said armatureessentially directly to said power distribution lines and having a gatedmeans to selectively supply power line voltage to said armature, saidcontrol means being connected to selectively gate said gated means.

4. The motor control circuit of claim 1, wherein said first meansestablishes an essentially constant direct current level at any givenmotor operation deviation, said second means includes a full waverectifying means establishing a direct current armature current relatedsignal having a full wave ripple component and includes means to impresssaid related signal on said direct current error signal to establish acontrol signal, and said motor control means including means toestablish an alternating current reference signal and establishing anoperating signal in synchronism with a selected intercept of the controlsignal with said reference signal.

5. A motor control circuit for operating a motor at a selected ratedvoltage from a transformer supply means comprising, command signal meansto establish a command signal proportional to a desired motor operationat a selected rated voltage, first feedback means to establish afeedback signal proportional to actual motor armature voltage, firstsummating means connected to said command signal means and to saidfeedback means to provide an error signal proportional to the algebraicsum of said command signal and said feedback signal, second feedbackmeans to establish a second feedback signal proportional to the armaturevoltage and a third negative feedback signal proportional to thearmature current, a second summating means connected to said firstsummating means and to said second feedback means to provide a summatedcontrol signal proportional to the algebraic sum of said error signaland said voltage and current feedback signals, said second and thirdfeedback signals being of opposite polarity to establish a compensatingsignal related to the armature voltage drop, control means connected tosaid second summating means to control the current supplied to thearmature, said third feedback signal establishing a continuouscompensating armature current signal in excess of 25 percent of ratedvoltage and establishing a limit on the armature current supplied to thearmature with the transfer function essentially independent of theimpedance of the motor armature and the transformer supply means.

6. The motor control circuit of claim 5, having means to inversely varythe relative magnitude of the error signal and the third feedback signalfor a given change in armature voltage and thereby vary the limit of thearmature current.

7. The motor control circuit of claim 5, having an adjustable resistancemeans connecting the first summating means to the second summating meansto vary the magnitude of the error signal for a given change in armaturevoltage and thereby vary the limit of the armature current.

8. The motor control circuit of claim 5, having a series connectedadjustable potentiometer connecting the first summating means to thesecond summating means to vary the limit of the armature current.

9. The motor control circuit of claim 5, having gated means including analternating current input and a periodic output, and gating meansconnecting the gated means to said second summating means andestablishing a gating signal in response to the intercept of analternating current reference voltage signal with the summated controlsignal from said second summating means.

10. The motor control circuit of claim 5, wherein said control meansincludes a gated rectifying means, including phase controlled rectifiershaving an altemating current input and a direct current output and afiring means responsive to the intercept of said summated control signaland a reference voltage signal in a selected phase relationship with thevoltage impressed upon said phase controlled rectifiers.

11. The motor control circuit of claim 5, wherein said control meansincluding a three-phase full-wave rectifying means having an alternatingcurrent input and individual gated rectifiers for sequentiallytransmitting the sequential power phases as a direct current, saidcontrol means including an alternating current firing means inpreselected phase relationship to said alternating current input, saidfirst feedback means establishing a direct current feedback signal, saidfirst summating means including an amplifier to provide a direct currenterror signal, said second feedback signal being a direct currentproportional to the armature voltage and the third feedback signal beingan unfiltered direct current signal proportional to current from a fullwave rectifying means, said second summating means including anamplifier establishing said summated control signal, and means toimpress said control signal on said firing means to control the firingof the gated rectifiers.

12. The motor control circuit of claim 11 wherein said armature currentsignal is of the order of 50 percent of rated voltage.

13. The motor control circuit of claim 11 wherein said armature currentsignal is in the range of 25 percent to percent of rated voltage.

1. A motor control circuit for operating a motor having an armature at aselected rated voltage comprising, first means to establish an errorsignal proportional to the difference between a desired motor operationat a selected rated voltage and the actual motor operation, second meansto establish a compensating signal proportional to thecounterelectromotive force of the armature of the motor, said last namedmeans including negative current feedback means establishing acompensating voltage signal directly proportional to the armaturecurrent and forming a single combination circuit impedance compensatingsignal and an armature current limit signal, said compensating voltagesignal being correspondingly scaled to the error signal and being equalto at least 30 percent of the corresponding scaled rated voltage toestablish and maintain a continuous and smoothly changing transferfunction including said compensating voltage signal, and motor controlmeans responsive to said error signal and to said compensating signal.2. The motor control circuit of claim 1, having supply lines connectedto said motor and said current feedback means establishes said transferfunction which is essentially independent of the resistance of the motorarmature, the impedance of the supply lines and motor speed.
 3. Themotor control circuit of claim 1, including three-phase powerdistribution lines, and rectifying means connecting said armatureessentially directly to said power distribution lines and having a gatedmeans to selectively supply power line voltage to said armature, saidcontrol means being connected to selectively gate said gated means. 4.The motor control circuit of claim 1, wherein said first meansestablishes an essentially constant direct current level at any givenmotor operation deviation, said second means includes a full waverectifying means establishing a direct current armature current relatedsignal having a full wave ripple component and includes means to impresssaid related signal on said direct current error signal to establish acontrol signal, and said motor control means including means toestablish an alternating current reference signal and establishing anoperating signal in synchronism with a selected intercept of the controlsignal with said reference signal.
 5. A motor control circuit foroperating a motor at a selected rated voltage from a transformer supplymeans comprising, command signal means to establish a command signalproportional to a desired motor operation at a selected rated voltage,first feedback means to establish a feedback signal proportional toactual motor armature voltage, first summating means connected to saidcommand signal means and to said feedback means to provide an errorsignal proportional to the algebraic sum of said command signal and saidfeedback signal, second feedback means to establish a second feedbacksignal proportional to the armature voltage and a third negativefeedback signal proportional to the armature current, a second summatingmeans connected to said first summating means and to said secondfeedback means to provide a summated control signal proportional to thealgebraic sum of said error signal and said voltage and current feedbacksignals, said second and third feedback signals being of oppositepolarity to establish a compensating signal related to the armaturevoltage drop, control means connected to said second summating means tocontrol the current supplied to the armature, said third feedback signalestablishing a continuous compensating armature current signal in excessof 25 percent of rated voltage and establishing a limit on the armaturecurrent supplied to the armature with the transfer function essentiallyindependent of the impedance of the motor armature and the transformersupply means.
 6. The motor control circuit of claim 5, having means toinversely vary the relative magnitude of the error signal and the thirdfeedback signal for a given change in armature voltage and thereby varythe limit of the armature current.
 7. The motor control circuit of claim5, having an adjustable resistance means connecting the first summatingmeans to the second summating means to vary the magnitude of the errorsignal for a given change in armature voltage and thereby vary the limitof the armature current.
 8. The motor control circuit of claim 5, havinga series connected adjustable potentiometer connecting the firstsummating means to the second summating means to vary the limit of thearmature current.
 9. The motor control circuit of claim 5, having gatedmeans including an alternating current input and a periodic output, andgating means connecting the gated means to said second summating meansand establishing a gating signal in response to the intercept of analternating current reference voltage signal with the summated controlsignal from said second summating means.
 10. The motor control circuitof claim 5, wherein said control means includes a gated rectifyingmeans, including phase controlled rectifiers having an alternatingcurrent input and a direct current output and a firing means responsiveto the intercept of said summated control signal and a reference voltagesignal in a selected phase relationship with the voltage impressed uponsaid phase controlled rectifiers.
 11. The motor control circuit of claim5, wherein said control means including a three-phase full-waverectifying means having an alternating current input and individualgated rectifiers for sequentially transmitting the sequential powerphases as a direct current, said control means including an alternatingcurrent firing means in preselected phase relationship to saidalternating current input, said first feedback means establishing adirect current feedback signal, said first summating means including anamplifier to provide a direct current error signal, said second feedbacksignal being a direct current proportional to the armature voltage andthe third feedback signal being an unfiltered direct current signalproportional to current from a full wave rectifying means, said secondsummating means including an amplifier establishing said summatedcontrol signal, and means to impress said control signal on said firingmeans to control the firing of the gated rectifiers.
 12. The motorcontrol circuit of claim 11 wherein said armature current signal is ofthe order of 50 percent of rated voltage.
 13. The motor control circuitof claim 11 wherein said armature current signal is in the range of 25percent to 80 percent of rated voltage.